What does he threaten?
For a large reach weapon It'd be 15-20ft out... But a medium?
Any thoughts?
By RAW a reach weapon doubles your natural reach. So there is no difference between a small, medium or large reach weapon regarding reach for a large creature. The type of the weapon changes (see inappropriately sized weapons) but the reach is still 15-20 ft.
To be honest.. the reach rules cause a lot a problems as soon as we talk about small, tiny or large(r) weapons and the use of inappropriately sized weapons.
Reach Weapons: Glaives, guisarmes, lances, longspears, ranseurs, and whips are reach weapons. A reach weapon is a melee weapon that allows its wielder to strike at targets that aren't adjacent to him. Most reach weapons double the wielder's natural reach, meaning that a typical Small or Medium wielder of such a weapon can attack a creature 10 feet away, but not a creature in an adjacent square. A typical Large character wielding a reach weapon of the appropriate size can attack a creature 15 or 20 feet away, but not adjacent creatures or creatures up to 10 feet away.
I can't find anywhere in the rules that addresses reach and inapropreately sized weapons together. However, how can a small longspear, about 4ft long, and a large longspear, about 16ft long, have the same reach?